The present invention relates to a driver circuit employing insulated gate field effect transistors (hereinafter abbreviated as IGFETs), and more particularly to a boot strap type driver circuit for a large capacity load which raises an output up to a power supply voltage.
As a circuit for providing a high level output equal to a power supply (V.sub.CC) level, a circuit employing a depletion type IGFET as a load of an output stage inverter has been known. Such type of circuits basically achieve a static operation and are not subjected to any restriction for timing of input signals. However, when the output is at a low level, the current flowing through the load was so large that they were not favorable in view of electric power consumption.
A so-called boot strap circuit has improved this shortcoming, and it can produce an output at a high level equal to a power supply lever despite its low power consumption. One example of the boot strap circuit is described in U.S. Pat. No. 3,898,479. In the boot strap circuit, the boot strap effect is achieved by controlling a charge operation of a boot strap capacitor over a delay time. Accordingly, the boot strap circuit cannot achieve the boot strap effect for every input waveform. In particular, when an input signal for providing a high level output is applied to the circuit employing N-channel IGFETs in succession to an input signal having a minute pulse width for providing a low level output, the electric charge stored in the boot strap capacitor is once discharged, and thereafter a sufficient charging period cannot be assured, so that the boot strap capacitor cannot be fully charged up and thus a sufficient boot strap effect is not achieved.
The above-described shortcoming that the output cannot always be held at a fixed level for any input condition, is a fatal defect for a circuit of high speed operation, especially for a circuit of static operation.